A radiometer is a device for measuring the radiation from a radiant energy source. See U.S. Pat. No. 4,801,202 entitled METHOD AND APPARATUS FOR RADIOMETER STAR SENSING issued on Jan. 31, 1989 to J. E. Wilcox and assigned to ITT Aerospace Optical. The invention described therein relates to a system for orienting earth scanning radiometer instruments by employing the instruments themselves to take star sightings to determine the exact optical axis of the instruments.
Conventional radiometers consist of an objective lens or mirror which collects radiation from a source and images it directly onto the surface of a detector capable of converting the incident radiation into an electrical signal.
To photograph images from either a polar or a geosynchronous orbit, conventional photoreconnaissance satellites utilize a radiometer having a scan mirror and a telescope. The scan mirror which looks out into space from the satellite platform, rotates to scan and thus attains an image of the radiant energy source of interest. The scan mirror reflects this radiant energy onto the telescopic mirrors of the radiometer. This radiant energy, which is then converted into an electrical signal, is recorded in data processors on board the satellite and can be relayed to controllers at ground stations on earth.
Collimators are optical devices which convert a diverging or converging beam of light into a collimated or parallel beam. Conventional collimators are used to monitor the optoelectronic performance of radiometers used in photoreconnaissance satellites by beam expanding and collimating radiant energy of a known intensity from either a laser or a blackbody source onto the scan mirror which then reflects this radiant energy onto the telescopic mirrors of the radiometer. The image formed by this radiant energy source is then recorded in data processors, enabling controllers at a ground station on earth to compare the most recently recorded images to previously recorded images and thus detect any problems with the optoelectronic performance of the radiometer. If any aberrations are found in the most recently recorded images, as compared to previously recorded images, the controllers can correct any such aberrations by using well known digital image processing techniques. Collimators are well known in the art. Examples of such devices are found in chapter 14.4 of a text by W. J. Smith entitled Modern Optical Engineering, which was published by McGraw-Hill in 1966.
Two shortcomings of conventional collimaters are that: they are designed and built with n consideration given to the size and weight constraints associated with satellite applications; and they do not include a means for monitoring the stability and accuracy of the radiant energy source.
A final disadvantage of conventional collimators is that they utilize an externally mounted sliding cover over the output aperture of the housing for the collimator. As the cover is externally mounted, it is therefore not flush with the housing and can, being in the field of the view of the scan mirror, cause vignetting of the image that is then reflected by the scan mirror onto the telescopic mirrors of the radiometer.
It is an object of this invention to provide a collimator which is smaller and lighter than conventional collimators and therefore more adaptable for satellite applications. The present invention accomplishes this by utilizing a single off-axis reflective mirror rather than the multiple on-axis mirrors used in conventional collimators.
It is a further object of this invention to provide an off-axis collimator which utilizes a broadband radiant energy source to monitor the optoelectronic performance of the radiometer.
It is also an object of this invention to provide an off-axis collimator which includes radiant energy detectors to monitor the optoelectronic performance of the radiant energy source.
Another objective of this invention is to provide a housing for an off-axis collimator which utilizes an inner rotating door to open and close the output aperture of the housing and thus minimize the volume of this off-axis collimator assembly.